1. Field of The Invention
The invention relates to a sequential buffer device for connection to a player for an optically readable memory disc, which player includes error protection members for data originating from the disc as well as reconfiguration members for presenting data elements of successive data frames on a serial output at a predetermined, uniform speed.
2. Description of the Related art
A memory disc of this kind is known as a "Compact disc" and serves for the storage of high-quality audio information. For the correction of incorrect bits, the data on the disc is stored according to two cross-interleaved Reed-Solomon codes. Random errors as well as error bursts can thus be corrected. In rare cases the correction will not be successful and a concealment strategy which is permissible for audio information is adopted, for example replacement of an incorrect value by an interpolated value.
It has been recognized that a disc of this kind can also be attractively used for the storage of a variety of other information, for example computer programs or dictionaries. In some cases, such a concealment strategy will then not make sense. However, the reliability of given information must still be higher than can be achieved by means of the discribed codes. To this end, a previously published system utilizes a so-called third layer of error protection which is based on so-called frames.
A frame may be composed of successively 12 bytes of synchronization data, 4 bytes of header data, 2048 bytes of user data and finally 288 bytes of inter alia error detection and error correction data. For a detailed description of the encoding and decoding of data on such a CD ROM reference is made to published British Patent Application No. 2,156,555.
A single disc of such a CD ROM offers a storage capacity of 600 Mbytes; its access time is also very short. Moreover, a CD ROM can be manufactured comparatively simply by means of the same equipment as used for the manufacture of audio Compact Discs, so that the cost of the CD ROM may be low. The player for a CD ROM may also be inexpensive, because such a player can have substantially the same construction as the CD players for audio applications which are already being manufactured in large numbers at low cost. One of the components thereof will be absent or inoperative, i.e., the digital-to-analog converter for forming a continuous audio output signal. Such a CD audio player supplies a data stream at a constant speed by way of a feedback mechanism to the disc drive. A similar storage format has already been proposed as a CD ROM for the storage of erasable and/or modifiable information. Alternatively, frames may alternately contain such error protection information. For the sake of brevity, all such storage formats will be referred to hereinafter as CD ROM. A common property thereof consists in that the third layer of error protection information need only rarely be activated, because the vast majority of the errors is corrected by the two cross-interleaved Reed-Solomon codes.
An audio player comprises two important sub-systems. The first sub-system provides the demodulation from channel bits to code bits and the error protection by way of the two cross-interleaved Reed-Solomon codes. The second sub-system provides reconfiguration, so that the sequence of the digital signals corresponds to the time sequence of the audio signal to be reproduced. This second sub-system may also implement the concealment strategy. The output of the second sub-system is then connected to the digital-to-analog converter. Both sub-system supply streams of digital of information at a uniform speed. Theredfore, the frames as defined supra may be derived from one of the two outputs. Both methods have their own specific advantages. Connection to the first sub-system (and omission of the second sub-system) offers a reconfiguration member which is to be optimized for CD ROM. Connection to the second sub-system, sometimes results in a more flexible set-up, because a selection can be made between audio reproduction and the supply of data frames.
Preferably the error protection for the third layer already takes place in the CD ROM player.
If the computer connected to the CD ROM player cannot take up the data read from the CD ROM at the same speed, or it an interruption occurs in the taking up of data, data is liable to be lost if the output of data by the CD player is not stopped in time. Such data output can be stopped in time by moving the laser pick-up device of the CD player back to the preceding track, but such an operation is also undesirable because such a movement is rather time-consuming inter alia because the data output must be synchronized again before the CD player can output data once more, fore example, because the correct header must be localized again.